CE 406 Structural Steel Design

Transcription

1 A. DESIGN REFERENCES: CE 406 Structural Steel Design International Building Code, 2009 Edition ASCE Minimum Design Loads for Buildings and Other Structures Manual of Steel Construction 14th Edition, American Institute of Steel Construction Building Code Requirements for Reinforced Concrete (ACI ), American Concrete Institute. Building Code Requirements for Masonry Structures (ACI ) and Specifications for Masonry Structures (ACI ), American Concrete Institute ASTM Standards, American Society for Testing and Materials Standard Specification for Open Web Joists, K-Series, LH-Series and Joist Girders, Steel Joist Institute SDI Specifications and Commentaries for Non-Composite Steel Form Deck and Steel Roof Deck, Steel Deck Institute B. DESIGN LOADS: All portions of the structure shall be designed in accordance with the following: VERTICAL LOADS ROOF Dead Loads: (estimated before design, verified after) (Self-weight and weights of supported equip & building materials) Typical roof: Single ply membrane attached with allowance for built-up roof = 5 psf Insulation = 5 psf Deck = 2 psf Framing (estimate) = 6 psf Mech. & Elect. (HVAC & lights) = 5 psf Misc. Hanging (ceiling & sprk) = 5 psf Total DL = 28 psf Live Loads: (variable location and magnitude) Live Load (non-reduced) LL = 20 psf Chapter 2 Page 1

2 ELEVATED FLOOR SLABS Dead Loads: (estimated before design, verified after) (Self-weight and weights of supported equip & building materials) 3" NWT Concrete on Form Deck = 36 psf Framing (estimate) = 10 psf Mech. & Elect. (HVAC & lights) = 5 psf Misc. Hanging (Sprinklers, Ceiling etc.) = 5 psf Floor DL = 56 psf Live Loads: (Floor LL based on Occupancy) Office Areas = 50 psf Public Rooms and Lobbies = 100 psf Equipment and Mechanical Areas = 150 psf SLAB-ON-GRADE Live Loads: (Industrial Floor) Uniform Live Load = 500 psf Actual Wheel Loads for a Forklift capable of carrying 2 (two) 2,000 pound pallets Concentrated Rack Storage post loads (see Rack Layout) ENVIRONMENTAL LOADS (based on location) (usually lateral) Snow Loads: Ground Snow Load = 10 psf Ce = 1.0 Exposure Factor (Exposure C, Partially Exposed) Ct = 1.0 Thermal Factor I = 1.2 Importance Factor Wind: Per International Building Code Basic Wind Speed, V = 90 mph Use Factor, I (Essential Facility) = 1.15 Seismic: S s = S 1 = Site Class = D Seismic Use Group = I Seismic Design Category = D Response Modification Coefficient, R = 5 (both directions) = (Other Loads :) (Traffic, Impact, Longitudinal, Soil, Thermal, Fluid, Blast ) Chapter 2 Page 2

3 Design Philosophies (AISC Manual 2-8 to 2-12) Limit States Design Philosophy Strength Serviceability Strength Safety Related (Flexural, Axial strength, Buckling, Fatigue, Fracture, Yielding) Focus of AISC specification Serviceability Performance (Concerns use & occupancy, deflection, vibration, cracking, deterioration) Allowable Strength Design (ASD) Members are sized to support Service (Unfactored) loads based on Allowable strength Typical: R a R n Ω Where: R a = Allowable Strength R n = Nominal Strength according to Specification provision Ω = Factor of Safety given by Specification, Typical range 1.50 to 2.00 Examples: Nominal moment strength Nominal axial compressive strength M a M n Ω F.O.S. = 1.67 (bending) P a P n Ω F.O.S. = 1.67 (compression) Actual Moment (Unfactored) Actual Axial Force (Unfactored) Chapter 2 Page 3

4 Allowable Strength R ΣR =R n i a Ω Required Strength determined from ASD Load Combinations Load Combination Equations D = dead load L = live load due to occupancy L r = roof live load S = snow load R = rain load exclusive of the ponding contribution W = wind load E = earthquake load ASD Equations 1. D 2. D + L (floor) 3. D + (L r or S or R) (roof) 4. D L (L r or S or R) (column) 5. D + (0.6W or 0.7E) 6a. D L (0.6W) (L r or S or R) 6b. D L (0.7E) (S) D + 0.6W D + 0.7E Note W and 0.7E reduction of wind and earthquake force which is already factored or ultimate Note in equations 4. and 6. Combination of time-variable loads (0.75 not taken on dead load) Ref: ASCE 7-10 p. 8 Steel Manual-14 th ed p Text p. 57 Chapter 2 Page 4

5 Load & Resistance Factor Design (LRFD) Members are sized to support Factored loads based on Reduced Material strength Typical: R R u n Load factor, typ. > 1.0 λ Q i i R n Nominal strength Resistance factor, typ. < 1.0 Working (service) load : Accounts for uncertainties in material strength, dimensions, workmanship (fabrication/ erection stresses), residual stresses (uneven cooling), also, the consequence of failure. λ: i Accounts for uncertainties in estimating magnitude (D = 1.2, L = 1.6) Dead estimate more accurate the live load estimate. Environmental loads Act of God/Nature, difficult to predict Earthquake and Wind (already ultimate/factored load), W = E = 1.0 (formerly 1.6) LRFD Equations D (only dead load) D + 1.6L +0.5(L r or S or R) (floor/column) D + 1.6(L r or S or R) + (L * or 0.5W) (roof/column) D + 1.0W + L * + 0.5(L r or S or R) (column/roof) D + 1.0E + L * + 0.2S D + 1.0W D + 1.0E Ref: ASCE 7-10 p. 7 Steel Manual-14 th ed p Text p. 53 * The load factor on L in combinations (3.), (4.), and (5.) is taken as 1.0 for floors in places of public assembly, for live loads in excess of 100 psf and for parking garage live load. The live factor is permitted to equal 0.5 for other live loads. Downward load (gravity) the maximum gravity load. is positive (+). Load combinations 1. through 5.account for Upward load (uplift) is negative (-). Load combinations 6. and 7. are used to account for the possibilities of uplift. Chapter 2 Page 5

6 Example #1 (Example 2-1, p. 55, text) 8-0 D = 50 psf (not including self-weight) L = 80 psf Find Governing Load, lbs / ft or plf D = 55 plf + (50 psf) 8 ft = 455 plf L = (80 psf) 8 ft = 640 plf 1. U = 1.4 (D) = 1.4 (455) = = 637 plf 2. U = 1.2 (D) (L) (L r or S or R) = 1.2 (455) (640) (0) = = 1570 plf controls Example #2 D = 20 psf S = 30 psf W = 32 psf (assume suction upward only) Determine U: 1. U = 1.4 (D) = 1.4 (20) = = 28 psf 2. U = 1.2 (D) (L) (L r or S or R) = 1.2 (20) (0) (30) = = 39 psf 3. U = 1.2 (D) (L r or S or R) + (0.5 L or 0.5 W) = 1.2 (20) (30) (-32) = = 56 psf or 1.2 (20) (30) = = 72 psf 4. U = 1.2 (D) (W) L (L r or S or R) = 1.2 (20) (-32) (0) (30) = = 7 psf or 1.2 (20) (30) = = 39 psf 5. U = 1.2 (D) (E) (L) (S) = 1.2 (20) (0) (0) (30) = = 30 psf 6. U = 0.9 (D) (W) = 0.9 (20) (-32) = = -14 psf 7. U = 0.9 (D) (E) = 0.9 (20) (0) = = 18 psf The roof should be designed for: 72 psf (downward) 3. (1.2D + 1.6S) -14 psf (upward) 6. (0.9D + 1.0W) Chapter 2 Page 6